The present invention relates to an image forming apparatus for forming an image by recording a pixel dot on a recording medium by each recording element arranged on a recording head, and forming a gradation image by changing a size of the pixel dot.
As for a conventional image forming apparatus of this kind, an image forming apparatus for forming a monochrome image by controlling recording of the pixel dot in response to resolution, is known. When an image having gradation such as a natural image is recorded by such an apparatus, the pixel dot is positioned by halftone processing such as the ordered dither method, the error diffusion method and the like, and shading of the image is expressed in accordance with the rate of area of the pixel dot in each certain unit area.
When such halftone processing is carried out, quality of the image depends on the resolution of the apparatus. If the apparatus has high resolution, many shades can be expressed in each certain unit area and the image quality having high gradation can be obtained. However, if the apparatus has low resolution, the number of shades in each certain unit area is reduced. For this reason, the unit area needs to be increased to increase the number of shades in each certain unit area and, as a result, apparent resolution is deteriorated.
Further, when an image is a character or a line drawing, the image quality also depends on the resolution of the apparatus. That is, when the apparatus has low resolution, serrate portions called jaggy are generated at hatched parts of a character or a line drawing, which causes deterioration of the image quality.
As a solution of this problem, an image forming apparatus capable of representing a pixel with gradation of a few levels by modulating the recording area of pixel dots in one pixel, is known. In a case of a natural image, the apparent resolution becomes high and smooth gradation can be reproduced, in such an apparatus which can express one pixel with gradation of a few levels. Further, in a case of an image of a character or a line drawing, it is possible to prevent the jaggy from becoming conspicuous by controlling the pixel area at the hatched portion. For example, in an electrophotographic type image forming apparatus using an LED line head on which a number of LEDs (light-emitting diodes) are arranged as recording elements, the light emitting time for one pixel is controlled, the recording position of small-size pixel dots can be controlled in one pixel by the effect of overlapping the optical energy distribution on a photosensitive member and it is thereby possible to prevent the jaggy from becoming conspicuous.
Some recording heads, however, are unable to move the recording position of the small-size pixel dots within one pixel as carried out in the ink-jet system. If the gradation is to be expressed by changing the pixel dot size with such a recording head, a space is made between pixel dots when small-size pixel dots are in close contact with one another, and blur may be thereby generated in the image quality. In order to prevent the generation of blur as much as possible, it is possible to prevent a space from being formed between the pixel dots by comparatively increasing the size of the maximum pixel dot or small pixel dots. However, if the size of the pixel dots at each gradation is determined, the gradation is good in a case of an image of a character or a line drawing while it is worsened in a case of a natural image since the determined size is different from an optimum size.
The object of the present invention is to provide an image forming apparatus using a recording head, which can carry out image formation excellent in gradation in a case of a natural image, image formation generating no jaggy or blur at hatched portions and the like in a case of an image of a character or a line drawing, and image formation with high image quality regardless of the kind of the image.
According to the invention described in claim 1, there is provided an image forming apparatus comprising: setting means for setting plural kinds of pairs of control signals to be used to change a size of the dot on the basis of gradation information, in response to the kind of an image to be formed on the recording medium; selecting means for selecting a desired pair of control signals set by the setting means in response to the kind of the image to be formed on the recording medium; and a head driving unit for changing a size of the pixel dot recorded on the recording medium by driving the recording head in accordance with one control signal corresponding to the gradation information, of the pair of control signals output from the selecting means.
According to the invention of claim 1, using a recording head, it is possible to make image formation excellent in gradation in a case of a natural image, image formation generating no jaggy or blur at hatched portions and the like in a case of an image of a character or a line drawing, and image formation with high image quality regardless of the kind of the image.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.